Use in paper coatings of a mixture of a secondary polymeric dispersion and of a primary dispersion of an emulsion polymer

ABSTRACT

The use in paper coatings is described of a mixture of an aqueous secondary dispersion of a polymer selected from polyalkylene carbonates, polyesters and polyethylene and an aqueous primary dispersion of an emulsion polymer having a glass transition temperature of below 50° C.

The present invention relates to the use in paper coatings of a mixtureof an aqueous secondary dispersion of certain polymers and of an aqueousprimary dispersion of an emulsion polymer useful as binder in papercoatings.

Paper coating binders typically utilize emulsion polymers based onsuitable monomers such as, for example, styrene, butadiene, acrylicesters, acrylonitrile, vinyl acetate and other monomers polymerizableusing emulsion polymerization technology. These binders are produced asso-called primary dispersions, i.e., directly in the form of aqueouspolymeric dispersions, but are frequently inconvenient and costly toproduce.

There is a whole series of polymers which, owing to their polymerproperties, might in principle likewise be useful as binders for papercoating, but they cannot be produced by emulsion polymerization.Examples include polyesters, polypropylene carbonate, polyethylene andother substances which are typically polymerized in organic solvents orin the absence of any solvent and are obtained as mostly high-viscosityresins or solutions in organic solvents. They also include somenatural-based resins as well as chemically modified natural resins, forexample modified rosins. Converting these resins into an aqueous form ofdispersion requires the technology of secondary dispersal, whichtypically involves emulsifiers and water being added under high shear toobtain a secondary dispersion. However, this manner of production isscarcely able to produce polymer particles on the order of below 200 nmwhich are particularly suitable for paper coating. The secondarydispersions are generally too coarse for paper coatings.

Furthermore, appreciable quantities of emulsifier are often needed tostabilize the secondary dispersion, adversely affecting paper coatingperformance characteristics. Therefore, secondary dispersions havehitherto achieved practically no industrial significance as binders forpaper coating. The coarse and emulsifier-rich secondary dispersions havea distinctly weaker pigment-binding capacity than the finer andemulsifier-lean emulsion polymers.

The problem addressed by the present invention was that of expanding thespectrum of polymers useful for paper coating and more particularly ofpolymers which are inexpensive and convenient to produce, and ofproviding alternative binders for paper coating compositions having veryhigh binding power.

It was found that mixtures of secondary dispersions of certain polymersnot produced by emulsion polymerization, with primary dispersions ofcertain emulsion polymers do not have the typically expecteddisadvantage of lower binding power of secondary dispersions to theexpected degree and therefore are useful as binders for paper coating.As described in the examples hereinbelow, synergistic effects can beevidenced over a wide mixing range.

The present invention provides for the use of a mixture of

-   (a) an aqueous secondary dispersion of at least one polymer selected    from the group consisting of polyalkylene carbonates, polyesters and    polyethylene, and-   (b) an aqueous primary dispersion of at least one emulsion polymer    having a glass transition temperature of below 50° C.,    in paper coatings.

The present invention also provides a paper coating compositioncomprising

-   (a) an aqueous secondary dispersion of at least one polymer selected    from the group consisting of polyalkylene carbonates, polyesters and    polyethylene,-   (b) an aqueous primary dispersion of at least one emulsion polymer    having a glass transition temperature of below 50° C., and    preferably in the range from −10 to +30° C., and-   (c) inorganic pigments.

The present invention also provides paper or card coated with a papercoating composition of the present invention.

The average diameter of polymeric particles can be measured byhydrodynamic chromatography (HDC). In HDC, a colloidal sample elutesfrom a size-exclusion separation column sorted according to hydrodynamicradius. The eluent comprises salt, nonionic surfactants and anionicsurfactants. Elution time is calibrated with PS calibration latices.Measurement range extends from 15 nm to 1200 nm—larger components arefiltered out and not detected. Diameter and weight fractions can bemeasured to an accuracy of 3%. The fractions are weighted using the UVabsorption at 254 nm.

The glass transition temperature can be determined as differentialscanning calorimetry midpoint temperature (ASTM D 3418-08).

The weight ratio of emulsion polymer to secondary dispersion polymer ispreferably in the range from 1:2 to 2:1.

The glass transition temperature of secondary dispersion polymers ispreferably in the range from −50 to +50° C., while any partialcrystallinity of polymers can lead to special effects (melting point).Suitable combinations of primary and secondary dispersions (e.g., lowglass transition temperature of primary dispersion and high glasstransition temperature of secondary dispersion) here expand the spectrumof possible polymers.

The average particle size of secondary dispersion polymers is preferablybelow 1 μm and more preferably below 400 nm, but not less than 50 nm.

Useful secondary dispersions include for example secondary dispersionsbased on polyalkylene carbonates and preferably based on polypropylenecarbonate. Unlike aqueous polymeric dispersions where polymer chainscomprise a backbone constructed of carbon atoms, aqueous dispersions ofpolyalkylene carbonates are not obtainable via emulsion polymerization.On the contrary, polymers of this type are generally prepared viapolycondensation of aliphatic diols and phosgene or via polyaddition ofaliphatic oxiranes onto CO₂ in the presence of suitable catalysts in anonaqueous medium and after solvent removal are typically obtained assolids. To obtain aqueous dispersions, they have to be dispersed inwater. Several processes are known for dispersing polyalkylenecarbonates in water. There is one type of process where high shearingforces are applied while the molten polymer is emulsified in the aqueousdispersing medium, which comprises surface-active substances, andsubsequently cooled down. Processes of this type are described in U.S.Pat. No. 4,320,041, DE 4115531, EP 1302402, EP 1514891 and WO 97/49762.In the other type of process, the polymer dissolved in an organic,preferably water-miscible solvent is mixed with the aqueous dispersingmedium and subsequently the organic solvent is removed. Processes ofthis type are described in U.S. Pat. No. 3,238,173, U.S. Pat. No.3,726,824 and WO 2007/074042. Further processes for producing secondarydispersions of polyalkylene carbonates are described in WO 2006/136555,PCT/EP2011/054471 and EP application numbered 11162705.5.

Preference for use as polyalkylene carbonates is given to aliphaticpolyalkylene carbonates that are predominantly constructed of repeatunits of formula (I). They may additionally further comprise repeatunits of formula (II):

—[O—(C═O)—O-A]—  (I)

—[O-A]—  (II)

where A is alkane-1,2-diyl of 2 to 10 carbon atoms orcycloalkane-1,2-diyl of 5 to 10 carbon atoms and may have differentmeanings within any one polymer. A is preferably selected fromalkane-1,2-diyl radicals, especially those of 2 to 4 carbon atoms, e.g.,1,2-ethanediyl, 1,2-propanediyl, 1,2-butanediyl,1-methyl-1,2-propanediyl and 2-methyl-1,2-propanediyl. In one specificembodiment of the present invention, A is 1,2-propanediyl to apredominant extent, i.e., to an extent of at least 70 mol % andespecially to an extent of at least 80 mol % or at least 90 mol %, basedon all repeat units. In this case, the aliphatic polycarbonate ispolypropylene carbonate. The proportion of carbonate repeat units offormula I in the polycarbonate is dependent on the reaction conditionsas well as particularly the catalyst used. In the preferredpolycarbonates, more than 80 mol % and preferably more than 90% of allrepeat units are repeat units of formula I.

Aliphatic polycarbonates are generally prepared by reacting aliphaticoxiranes, i.e., alkylene oxides, having in general from 2 to 10 carbonatoms or cycloalkylene oxides having in general from 5 to 10 carbonatoms, with CO2 in the presence of one or more suitable catalysts, seefor example Inoue, Makromol. Chem., Rapid Commun. 1, 775 (1980), Soga etal., Polymer Journal, 1981, 13, 407-10, U.S. Pat. No. 4,789,727 and U.S.Pat. No. 7,304,172. Suitable catalysts are in particular zinc and cobaltcatalysts as described for example in the aforementioned references andespecially in U.S. Pat. No. 4,789,727 and U.S. Pat. No. 7,304,172.Examples of suitable polyalkylene carbonates are the polyethylenecarbonates known from EP-A 1264860, which are obtained bycopolymerization of ethylene oxide and carbon dioxide in the presence ofsuitable catalysts, and especially polypropylene carbonate (see WO2007/125039 for example), obtainable by copolymerization of propyleneoxide and carbon dioxide in the presence of suitable catalysts. Thepolymer is also commercially available, for example from EmpowerMaterials Inc. or Aldrich.

The number average molecular weight Mn of polyalkylene carbonates andespecially of polypropylene carbonates is generally in the range from5000 to 500 000 daltons and especially in the range from 10 000 to 250000 daltons. Weight average molecular weight Mw is then typically in therange from 7000 to 5 000 000 daltons and especially in the range from 15000 to 2 000 000 daltons. In one specific embodiment of the presentinvention, the number average molecular weight Mn of polypropylenecarbonates is in the range from 50 000 to 100 000 daltons andspecifically in the range from 70 000 to 90 000 daltons. Weight averagemolecular weight Mw is then typically in the range from 100 000 to 500000 daltons and especially in the range from 150 000 to 400 000 daltons.The proportion of carbonate repeat units included in the total amount ofcarbonate and ether repeat units in the polymer is generally at least 80mol %, especially 90 mol %. Polydispersity (ratio of weight average (MW)to number average (MN)) is generally between 1 and 80 and preferablybetween 2 and 10. The polypropylene carbonates used may comprise up to1% of carbamate and urea groups.

Useful aliphatic polycarbonates also include chain-extended polyalkylenecarbonates. Chain extenders used for polyalkylene carbonates areespecially maleic anhydride, acetic anhydride, di- or polyisocyanates,di- or polyoxazolines or -oxazines or di- or polyepoxides. Examples ofisocyanates are aromatic diisocyanates such as toluoylene2,4-diisocyanate, toluoylene 2,6-diisocyanate, 2,2′-diphenylmethanediisocyanate, 2,4′-diphenylmethane diisocyanate, 4,4′-diphenylmethanediisocyanate, naphthylene 1,5-diisocyanate or xylylene diisocyanate andaliphatic diisocyanates such as especially 1,6-hexamethylenediisocyanate, isophorone diisocyanate ormethylenebis(4-isocyanatocyclohexane). Particular preference is given toaliphatic diisocyanates and of these especially to isophoronediisocyanate and particularly 1,6-hexamethylene diisocyanate. Usefulbisoxazolines include 2,2′-bis(2-oxazoline), bis(2-oxazolinyl)methane,1,2-bis(2-oxazolinyl)ethane, 1,3-bis(2-oxazolinyl)propane or1,4-bis(2-oxazolinyl)butane, especially 1,4-bis(2-oxazolinyl)benzene,1,2-bis(2-oxazolinyl)benzene or 1,3-bis(2-oxazolinyl)benzene. Chainextenders are preferably used in amounts of 0.01% to 5%, more preferably0.05% to 2% and even more preferably 0.08% to 1% by weight, based on thepolycarbonate quantity. Chain-extended polyalkylene carbonates typicallyhave a number average molecular weight Mn in the range from 30 000 to500 000 daltons, preferably in the range from 35 000 to 250 000 daltonsand more preferably in the range from 40 000 to 150 000 daltons.

Useful secondary dispersions include for example secondary dispersionsbased on polyesters. The polyesters are preferably thermoplasticpolymers comprising a multiplicity of ester groups and/or carbonategroups in the polymer backbone and having an acid number of preferablynot more than 10 mg of KOH/g, and that includes biodegradablepolyesters. Unlike aqueous polymeric dispersions where polymer chainscomprise a backbone constructed of carbon atoms, aqueous dispersions ofpolymers comprising a multiplicity of ester groups and/or carbonategroups in the polymer backbone are generally not obtainable via anemulsion polymerization process. On the contrary, it is usuallynecessary to prepare such polymers by way of a polycondensation and toconvert them subsequently into an aqueous dispersion, i.e., into aso-called secondary dispersion. There are several ways of doing this inprinciple. First, the dissolved polymer in organic, preferablywater-miscible solvent can be mixed with the aqueous dispersing mediumand the organic solvent removed again. Polymers having a high acidnumber can in turn be emulsified in water by rendering the aqueousdispersing medium alkaline with a base in order thereby to deprotonatethe carboxyl groups and thereby to further the self-emulsification ofthe polymer. Such a procedure is described in WO 98/12245 for example.Further ways to prepare polyester secondary dispersions are described inEP 1302502 A1, US 2005/058712, US 2002/0076639, U.S. Pat. No. 6,521,679and WO 2011/117308.

The polyesters typically have a number average molecular weight MN inthe range from 5000 to 1 000 000 daltons, especially in the range from8000 to 800 000 daltons and specifically in the range from 10 000 to 500000 daltons. The weight average molecular weight Mw of polyesters isgenerally in the range from 20 000 to 5 000 000 daltons, frequently inthe range from 30 000 daltons to 4 000 000 daltons and especially in therange from 40 000 to 2 500 000 daltons. The polydispersity index MW/MNis generally at least 2 and frequently in the range from 3 to 20 andespecially in the range from 5 to 15. Molecular weight andpolydispersity index can be determined via gel permeation chromatography(GPC) as per DIN 55672-1 for example. The viscosity number ofpolyesters, which is indirect measure of the molecular weight, istypically in the range from 50 to 500 ml/g, frequently in the range from80 to 300 ml/g and especially in the range from 100 to 250 ml/g(determined to EN ISO 1628-1 at 25° C. on a 0.5% by weight solution ofpolymer in 1:1 (w/w) o-dichlorobenzene/phenol.

Polyesters used may be amorphous or partly crystalline, branched orunbranched.

An aliphatic polyester is a polyester constructed exclusively fromaliphatic monomers. An aliphatic copolyester is a polyester constructedexclusively from at least two and especially at least three aliphaticmonomers, wherein the acid component and/or the alcohol componentpreferably comprises at least two mutually different monomers. Analiphatic-aromatic copolyester is a polyester constructed not only fromaliphatic monomers but also from aromatic monomers, wherein the acidcomponent preferably comprises at least one aliphatic acid and at leastone aromatic acid.

Aliphatic polyesters and copolyesters are particularly polylactides,polycaprolactone, block copolymers of polylactide with poly(C2-C4alkylene glycol), block copolymers of polycaprolactone with poly(C2-C4alkylene glycol) and also the hereinbelow defined copolyesters which areconstructed from at least one aliphatic or cycloaliphatic dicarboxylicacid or an ester-forming derivative thereof and at least one aliphaticor cycloaliphatic diol component and also optionally further components.

Copolyesters, especially aliphatic or aliphatic-aromatic copolyesters,constructed from at least one aliphatic or cycloaliphatic dicarboxylicacid or an ester-forming derivative thereof and at least one aliphaticor cycloaliphatic diol component and optionally one or more aromaticdicarboxylic acids or their ester-forming derivatives or mixturesthereof and also optionally further components are also suitable.

Useful aromatic dicarboxylic acids are generally aromatic dicarboxylicacids having 8 to 12 carbon atoms and preferably aromatic dicarboxylicacids having 8 carbon atoms. Examples are terephthalic acid, isophthalicacid, 2,6-naphthoic acid and 1,5-naphthoic acid and also ester-formingderivatives thereof. Especially the di-C1-C6-alkyl esters, e.g.,dimethyl, diethyl, diethyl, di-n-propyl, diisopropyl, di-n-butyl,diisobutyl, di-t-butyl, di-n-pentyl, diisopentyl or di-n-hexyl esters,are suitable. The anhydrides of dicarboxylic acids a2 are similarlysuitable ester-forming derivatives. In principle, however, it is alsopossible to use aromatic dicarboxylic acids having a larger number ofcarbon atoms, for example up to 20 carbon atoms. Aromatic dicarboxylicacids or ester-forming derivatives thereof can be used singly or asmixture of two or more thereof. Particular preference is given to usingterephthalic acid or its ester-forming derivatives such as dimethylterephthalate.

In general, the diols are selected among branched or linear alkanediolshaving 2 to 12 carbon atoms, preferably 4 to 8 or especially 6 carbonatoms, or cycloalkanediols having 5 to 10 carbon atoms. Examples ofsuitable alkanediols are ethylene glycol, 1,2-propanediol,1,3-propanediol, 1,2-butanediol, 1,4-butanediol, 1,5-pentanediol,2,4-dimethyl-2-ethylhexane-1,3-diol, 2,2-dimethyl-1,3-propanediol,2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-isobutyl-1,3-propanediol,2,2,4-trimethyl-1,6-hexanediol, especially ethylene glycol,1,3-propanediol, 1,4-butanediol and 2,2-dimethyl-1,3-propanediol(neopentylglycol); cyclopentanediol, 1,4-cyclohexanediol,1,2-cyclohexanedimethanol, 1,3-cyclohexanedimethanol,1,4-cyclohexanedimethanol or 2,2,4,4-tetramethyl-1,3-cyclobutanediol.Mixtures of different alkanediols can also be used. In thesecopolyesters, diol component B is preferably selected among C2-C12alkanediols and mixtures thereof. Preference is given to 1,3-propanedioland especially 1,4-butanediol.

Terephthalic acid and the aliphatic dicarboxylic acid can be used asfree acid or as ester-forming derivatives. Useful ester-formingderivatives include especially the di-C1-C6-alkyl esters, e.g.,dimethyl, diethyl, di-n-propyl, diisopropyl, di-n-butyl, diisobutyl,di-tert-butyl, di-n-pentyl, diisopentyl or di-n-hexyl esters. Anhydridesof dicarboxylic acids can likewise be used. The diol is preferably1,4-butanediol.

Useful polymers for secondary dispersions also include hydrocarbonwaxes, for example hydrocarbon waxes produced via free-radicalpolymerization in the high-pressure process or in the presence oforganometallic catalysts in the low-pressure process, and having anaverage molar mass range of 2000-20 000 (mass average), especiallypolyethylene waxes.

Oxidation with air or oxygen gives more polar oxidized PE waxes(polyethylene wax oxidates) which are somewhat easier to convert withsurfactants into aqueous wax dispersions.

Suitable secondary dispersions are obtainable either by precipitation ofwax dissolved in hot solvent or vegetable oil, by controlled coolingunder agitation (precipitated waxes, typical particle size 0.5 to 10 μm)or by emulsification of molten hot wax in water with subsequent cooling.The particle size of latter wax preparations is generally in the regionaround 100 nm.

Useful secondary dispersions include for example secondary dispersionsbased on polyethylene and are available as polyethylene wax emulsionsunder the designation Poligen®, e.g. Poligen® WE1 or Poligen® WE6 with aparticle size of about 100 nm and a molecular weight between 2700 and 11000. The dispersions have a solids content of about 35% coupled withviscosities <500 mPas (Brookfield).

The glass transition temperature of primary dispersion polymers ispreferably in the region of below 50° C. and more preferably in therange from −30 to +30° C.

The average size of primary dispersion polymer particles is preferablybelow 200 nm and more preferably in the range from 80 to 160 nm.

Polymers useful as binders in the primary dispersion are obtainable asemulsion polymer via free-radically initiated emulsion polymerizationfrom one or more ethylenically unsaturated, free-radically polymerizablemonomers in the presence or absence of a chain transfer agentcomposition. The polymeric binders have a glass transition temperatureTg of less than 50° C. and preferably below 30° C. The glass transitiontemperature can be determined as differential scanning calorimetrymidpoint temperature (ASTM D 3418-08).

Useful ethylenically unsaturated, free-radically polymerizable monomersmay be selected from the group consisting of vinylaromatic compounds,conjugated aliphatic dienes, ethylenically unsaturated acids,ethylenically unsaturated carboxamides, ethylenically unsaturatedcarbonitriles, vinyl esters of saturated C₁ to C₂₀ carboxylic acids,esters of acrylic acid or methacrylic acid with monohydric C₁ to C₂₀alcohols, allyl esters of saturated carboxylic acids, vinyl ethers,vinyl ketones, dialkyl esters of ethylenically unsaturated dicarboxylicacids, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide,N,N-dialkylaminoalkylacrylamides, N,N-dialkylaminoalkylmethacrylamides,N,N-dialkylaminoalkyl acrylates, N,N-dialkylaminoalkyl methacrylates,vinyl halides, aliphatic hydrocarbons having 2 to 8 carbon atoms and oneor two double bonds, or mixtures thereof.

The emulsion polymer consists to an extent which is preferably at least40% by weight, more preferably at least 60% by weight and even morepreferably at least 80% by weight of so-called principal monomers.Principal monomers are selected from C1-C20 alkyl (meth)acrylates, vinylesters of carboxylic acids comprising up to 20 carbon atoms,vinylaromatics having up to 20 carbon atoms, ethylenically unsaturatednitriles, vinyl halides, vinyl ethers of alcohols comprising 1 to 10carbon atoms, aliphatic hydrocarbons having 2 to 8 carbon atoms and oneor two double bonds, or mixtures thereof. Examples include alkyl(meth)acrylates having a C1-C10 alkyl moiety, such as methylmethacrylate, methyl acrylate, n-butyl acrylate, ethyl acrylate and2-ethylhexyl acrylate. Mixtures of alkyl (meth)acrylates are alsosuitable in particular. Vinyl esters of carboxylic acids having 1 to 20carbon atoms include, for example, vinyl laurate, vinyl stearate, vinylpropionate, vinyl versatate and vinyl acetate. Useful vinylaromaticcompounds include vinyltoluene, α-methylstyrene, p-methylstyrene,α-butylstyrene, 4-n-butylstyrene, 4-n-decylstyrene and preferablystyrene. Examples of nitriles are acrylonitrile and methacrylonitrile.Vinyl halides are chlorine-, fluorine- or bromine-substitutedethylenically unsaturated compounds, preferably vinyl chloride andvinylidene chloride. Specific examples of vinyl ethers are vinyl methylether and vinyl isobutyl ether. Preference is given to vinyl ethers ofalcohols comprising 1 to 4 carbon atoms. Specific examples ofhydrocarbons having 2 to 8 carbon atoms and one or two olefinic doublebonds are ethylene, propylene, butadiene, isoprene and chloroprene.

Preferred principal monomers are C1-C10 alkyl (meth)acrylates andmixtures thereof with vinylaromatics, more particularly styrene (alsoreferred together as polyacrylate binders) or hydrocarbons having 2double bonds, more particularly butadiene, or mixtures of suchhydrocarbons with vinylaromatics, more particularly styrene (alsoreferred together as polybutadiene binders). In the case ofpolybutadiene binders, the weight ratio of butadiene to vinylaromatics(more particularly styrene) can be for example between 10:90 to 90:10,more particularly 20:80 to 80:20. Polybutadiene binders are particularlypreferred.

In addition to the principal monomers, the polymer may comprise furthermonomers, for example monomers having carboxylic acid, sulfonic acid orphosphonic acid groups. Preference is given to carboxylic acid groups.Specific examples are acrylic acid, methacrylic acid, itaconic acid,maleic acid or fumaric acid and aconitic acid. The level ofethylenically unsaturated acids in the emulsion polymer is generallybelow or equal to 10% by weight, for example from 0.1% to 10% by weight.Further monomers include, for example, hydroxyl-containing monomers,more particularly C1-C10 hydroxyalkyl (meth)acrylates, or amides such as(meth)acrylamide.

In one embodiment of the present invention the emulsion polymer isconstructed from butadiene or mixtures of butadiene and styrene to anextent of at least 60% by weight or from C1 to C20 alkyl (meth)acrylatesor mixtures of C1 to C20 alkyl (meth)acrylates and styrene to an extentof at least 60% by weight.

Preferred polymeric binders are

-   (a) copolymers from (a1) 19.8 to 80 parts by weight of at least one    vinylaromatic compound, preferably styrene or methylstyrene, (a2)    19.8 to 80 parts by weight of at least one conjugated aliphatic    diene, preferably butadiene, (a3) 0.1 to 10 parts by weight of at    least one ethylenically unsaturated acid, preferably acrylic acid    and/or methacrylic acid, and (a4) 0 to 20 parts by weight of at    least one other monoethylenically unsaturated monomer, wherein the    parts by weight of monomers (a1) to (a4) sum to 100;-   (b) copolymers from (b1) 19.8 to 80 parts by weight of at least one    vinylaromatic compound, preferably styrene or methylstyrene, (b2)    19.8 to 80 parts by weight of at least one acrylate monomer selected    from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates,    preferably methyl acrylate, ethyl acrylate, n-butyl acrylate,    ethylhexyl acrylate, propylheptyl acrylate or their mixture, (b3)    0.1 to 10 parts by weight of at least one ethylenically unsaturated    acid, preferably acrylic acid and/or methacrylic acid, and (a4) 0 to    20 parts by weight of at least one other monoethylenically    unsaturated monomer, wherein the parts by weight of monomers (a1) to    (a4) sum to 100;-   (c) copolymers from vinyl acetate and at least one (meth)acrylate    monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl    methacrylates, and-   (d) ethylene/vinyl acetate copolymers.

One embodiment of the present invention utilizes as monomers

-   (A1) 19.8 to 80 parts by weight, preferably 25 to 70 parts by    weight, of at least one vinylaromatic compound,-   (B1) 19.8 to 80 parts by weight, preferably 25 to 70 parts by    weight, of at least one conjugated aliphatic diene,-   (C1) 0.1 to 15 parts by weight of at least one ethylenically    unsaturated acid, and-   (D1) 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight,    of at least one further monoethylenically unsaturated monomer other    than said monomers (A1) to (C1),    wherein the parts by weight of monomers (A1) to (D1) sum to 100.

One embodiment of the present invention utilizes as monomers

-   (A2) 19.8 to 80 parts by weight, preferably 25 to 70 parts by    weight, of at least one vinylaromatic compound,-   (B2) 19.8 to 80 parts by weight, preferably 25 to 70 parts by    weight, of at least one monomer selected from C1 to C18 alkyl esters    of acrylic acid and C1 to C18 alkyl esters of methacrylic acid,-   (C2) 0.1 to 15 parts by weight of at least one ethylenically    unsaturated acid, and-   (D2) 0 to 20 parts by weight, preferably 0.1 to 15 parts by weight,    of at least one further monoethylenically unsaturated monomer other    than said monomers (A2) to (02),    wherein the parts by weight of monomers (A2) to (D2) sum to 100.

The monomers of group (A1)/(A2) are vinylaromatic compounds, for examplestyrene, α-methylstyrene and/or vinyltoluene and their mixture. Of thisgroup of monomers, styrene is preferred. 100 parts by weight of totalmonomer mixtures used in the polymerization comprise for example from19.8 to 80 parts by weight and preferably from 25 to 70 parts by weightof at least one monomer of group (A1)/(A2).

Examples of monomers of group (B1) are 1,3-butadiene, isoprene,1,3-pentadiene, dimethyl 1,3-butadiene and cyclopentadiene. Of thisgroup of monomers, 1,3-butadiene and/or isoprene are preferred. 100parts by weight of monomer mixtures used altogether in the emulsionpolymerization comprise for example from 19.8 to 80 parts by weight,preferably from 25 to 70 parts by weight and especially from 25 to 60parts by weight of at least one monomer of group (B1).

Examples of monomers of group (C1)/(C2) are ethylenically unsaturatedcarboxylic acids, ethylenically unsaturated sulfonic acids andvinylphosphonic acids and salts thereof. Ethylenically unsaturatedcarboxylic acids used are preferably α,β-monoethylenically unsaturatedmono- and dicarboxylic acids having 3 to 6 carbon atoms in the molecule.Examples thereof are acrylic acid, methacrylic acid, itaconic acid,maleic acid, fumaric acid, crotonic acid, vinylacetic acid andvinyllactic acid. Useful ethylenically unsaturated sulfonic acidsinclude for example vinylsulfonic acid, styrenesulfonic acid,acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate andsulfopropyl methacrylate. Particular preference is given to acrylic acidand methacrylic acid, especially acrylic acid. The group (C1)/(C2)monomers comprising acid groups may be used in the polymerization asfree acids and also after partial or complete neutralization withsuitable bases. Aqueous sodium hydroxide solution, aqueous potassiumhydroxide solution or ammonia is preferably used as neutralizing agent.100 parts by weight of monomer mixtures used in the emulsionpolymerization comprise for example from 0.1 to 15 parts by weight,preferably from 0.1 to 10 parts by weight or from 1 to 8 parts by weightof at least one monomer of group (C1)/(C2).

Useful monomers of group (B2) include esters of acrylic aid andmethacrylic acid with monohydrate C₁ to C₁₈ alcohols such as methylacrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate,n-propyl acrylate, n-propyl methacrylate, isopropyl acrylate, isopropylmethacrylate, n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate,isobutyl methacrylate, sec-butyl acrylate, sec-butyl methacrylate,tert-butyl acrylate, tert-butyl methacrylate, pentyl acrylates, pentylmethacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate. 100parts by weight of total monomer mixtures used in the polymerizationcomprise for example from 19.8 to 80 parts by weight and preferably from25 to 70 parts by weight of at least one monomer of group (B2).

Monomers of group (D2) are other monoethylenically unsaturatedcompounds. Examples thereof are ethylenically unsaturated carboxamidessuch as more particularly acrylamide and meth-acrylamide, ethylenicallyunsaturated carbonitriles such as more particularly acrylonitrile andmethacrylonitrile, vinyl esters of saturated C₁ to C₁₈ carboxylic acids,preferably vinyl acetate, allyl esters of saturated carboxylic acids,vinyl ethers, vinyl ketones, dialkyl esters of ethylenically unsaturateddicarboxylic acids, N-vinylpyrrolidone, N-vinylpyrrolidine,N-vinylformamide, N,N-dialkylaminoalkylacrylamides,N,N-dialkylaminoalkylmethacrylamides, N,N-dialkylaminoalkyl acrylates,N,N-dialkylaminoalkyl methacrylates, vinyl chloride and vinylidenechloride. Useful monomers of group (D1) include the monomers of group(D2) and also esters of acrylic acid and of methacrylic acid withmonohydric C₁ to C₁₈ alcohols such as methyl acrylate, methylmethacrylate, ethyl acrylate, ethyl methacrylate, n-propyl acrylate,n-propyl methacrylate, isopropyl acrylate, isopropyl methacrylate,n-butyl acrylate, n-butyl methacrylate, isobutyl acrylate, isobutylmethacrylate, sec-butyl acrylate, sec-butyl methacrylate, tert-butylacrylate, tert-butyl methacrylate, pentyl acrylates, pentylmethacrylates, 2-ethylhexyl acrylate, 2-ethylhexyl methacrylate. Thisgroup of monomers is optionally used to modify the polymers. 100 partsby weight of monomer mixtures used in the emulsion polymerizationcomprise for example from 0 to 20 parts by weight or from 0.1 to 15parts by weight and especially from 0.5 to 10 parts by weight of atleast one monomer of group (D1)/(D2).

In one embodiment of the present invention, the further monomers (D1)and (D2) are each used in amounts of 0.1-15 parts by weight; thevinylaromatic compound is selected from styrene, methylstyrene and theirmixture; the conjugated aliphatic diene is selected from 1,3-butadiene,isoprene and their mixture; and the ethylenically unsaturated acid isselected from one or more compounds of the group consisting of acrylicacid, methacrylic acid, itaconic acid, maleic acid, fumaric acid,crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid,styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropylacrylate, sulfopropyl methacrylate, vinylphosphonic acid and saltsthereof.

The emulsion polymerization typically uses initiators that form freeradicals under the reaction conditions. Initiators are used for examplein amounts up to 2% by weight, preferably at not less than 0.9% byweight, for example in the range from 1.0% to 1.5% by weight, based onthe monomers to be polymerized. Suitable polymerization initiatorsinclude, for example, peroxides, hydroperoxides, hydrogen peroxide,sodium persulfate, potassium persulfate, redox catalysts and azocompounds such as 2,2-azobis(4-methoxy-2,4-dimethylvaleronitrile),2,2-azobis(2,4-dimethylvaleronitrile) and 2,2-azobis(2-amidinopropane)dihydrochloride. Examples of further suitable initiators are dibenzoylperoxide, tert-butyl perpivalate, tert-butyl per-2-ethylhexanoate,di-tert-butyl peroxide, diamyl peroxide, dioctanoyl peroxide, didecanoylperoxide, dilauroyl peroxide, bis(o-tolyl) peroxide, succinyl peroxide,tert-butyl peracetate, tert-butyl permaleate, tert-butyl perisobutyrate,tert-butyl perpivalate, tert-butyl peroctoate, tert-butyl perbenzoate,tert-butyl hydroperoxide, azobisisobutyronitrile,2,2″-azobis(2-methylbutyronitrile),2,2″-azobis(2,4-dimethylvaleronitrile) and2,2″-azobis(N,N″-dimethyleneisobutyroamidine) dihydrochloride.Initiators are preferably selected from the group consisting ofperoxodisulfates, peroxosulfates, azo initiators, organic peroxides,organic hydroperoxides and hydrogen peroxide. Particular preference isgiven to using water-soluble initiators, for example sodium persulfate,potassium persulfate, ammonium persulfate, sodium peroxodisulfate,potassium peroxodisulfate and/or ammonium peroxodisulfate. Thepolymerization can also be initiated by means of high-energy rays suchas electron beams or irradiation with UV light.

The amount of chain transfer agents is for example in the range from0.01% to 5% and preferably in the range from 0.1% to 1% by weight, basedon the monomers used in the polymerizetion. The chain transfer agentsare preferably added together with the monomers. However, they can alsobe partly or wholly present in the initial charge. They can also beadded in stages at different times to the monomers.

To augment the dispersal of the monomers in the aqueous medium, theprotective colloids and/or emulsifiers customarily used as dispersantscan be used. A detailed description of suitable protective colloids isgiven in Houben-Weyl, Methoden der organischen Chemie, volume XIV/1,Makromolekulare Stoffe, Georg-Thieme-Verlag, Stuttgart, 1961, pages 411to 420. Suitable emulsifiers include surface-active substances whosenumber average molecular weight is typically below 2000 g/mol orpreferably below 1500 g/mol, while the number average molecular weightof the protective colloids is above 2000 g/mol, for example in the rangefrom 2000 to 100 000 g/mol and more particularly in the range from 5000to 50 000 g/mol. Suitable emulsifiers include, for example, ethoxylatedC₈ to C₃₆ fatty alcohols having a degree of ethoxylation in the rangefrom 3 to 50, ethoxylated mono-, di- and tri-C₄-C₁₂-alkylphenols havinga degree of ethoxylation in the range from 3 to 50, alkali metal saltsof dialkyl esters of sulfosuccinic acid, alkali metal and ammonium saltsof C₈ to C₁₂ alkyl sulfates, alkali metal and ammonium salts of C₁₂ toC₁₈ alkylsulfonic acids and alkali metal and ammonium salts of C₉ to C₁₈alkylarylsulfonic acids. Cation-active emulsifiers are, for example,compounds having at least one amino or ammonium group and at least oneC₈ to C₂₂ alkyl group. When emulsifiers and/or protective colloids areused as auxiliaries to disperse the monomers, the amounts used thereofare for example in the range from 0.1% to 5% by weight, based on themonomers.

Useful protective colloids include for example degraded starch,especially maltodextrin. Useful starting starches for preparing thedegraded starches include all native starches such as starches frommaize (corn), wheat, oats, barley, rice, millet, potatoes, peas,tapioca, sorghum or sago. Also of interest are those natural starcheswhich have a high amylopectin content such as wax maize starch and waxpotato starch. The amylopectin content of these starches is above 90%,usually in the range from 95 to 100%. Starches modified chemically byetherification or esterification can also be used for preparing thepolymer dispersions of the present invention. Such products are knownand commercially available. They are prepared for example byesterification of native starch or degraded native starch with inorganicor organic acids, their anhydrides or chlorides. Of particular interestare phosphated and acetylated degraded starches. The most common methodto etherify starches consists in treating starch with organic halogencompounds, epoxides or sulfates in aqueous alkaline solution. Knownstarch ethers are alkyl ethers, hydroxyalkyl ethers, carboxyalkyl ethersand allyl ethers. The reaction products of starches with2,3-epoxypropyltrimethylammonium chloride are also useful. Particularpreference is given to degraded native starches, more particularlynative starches degraded to maltodextrin. Further suitable starchesinclude cationically modified starches, i.e., starch compounds havingamino groups or ammonium groups. The degraded starches have for examplean intrinsic viscosity η_(i) of less than 0.07 dl/g or less than 0.05dl/g. The intrinsic viscosity η_(i) of the degraded starches ispreferably in the range from 0.02 to 0.06 dl/g. The intrinsic viscosityη_(i) is determined in accordance with DIN EN1628 at a temperature of23° C.

In one embodiment of the present invention, the emulsion polymerizationis carried out in the presence of seed particles. The initial chargethen comprises polymer seed, especially a polystyrene seed, i.e., anaqueous dispersion of finely divided polymer, preferably polystyrene,having a particle diameter of 20 to 40 nm.

The emulsion polymerization takes place in an aqueous medium. Theaqueous medium may comprise for example completely ion-free water orelse mixtures of water and a miscible solvent such as methanol, ethanolor tetrahydrofuran. As soon as the particular polymerization temperaturedesired is reached or within the time span of 1 to 15 minutes,preferably 5 to 15 minutes after reaching the polymerizationtemperature, the metered addition of the monomers is commenced. They canbe for example pumped into the reactor continuously within for example60 minutes to 10 hours, usually within 2 to 4 hours. Preferably, thereaction mixture in the initial charge is heated to the requisitetemperature at which the polymerization proceeds. These temperatures arefor example from 80 to 130° C. and preferably from 85 to 120° C. Thepolymerization can also be carried out under superatmospheric pressure,for example at pressures up to 15 bar, e.g., at 2 to 10 bar. Monomeraddition can take the form of a batch, continuous or staged operation.

After the polymerization has ended, further initiator may optionally beadded to the reaction mixture and a postpolymerization performed at thesame temperature as the main polymerization or else at a lower or highertemperature. To complete the polymerization reaction, it will in mostcases suffice to stir the reaction mixture at the polymerizationtemperature for example 1 to 3 hours after addition of all the monomers.The pH in the polymerization can be for example in the range from 1 to5. After polymerization, the pH is adjusted to a value of between 6 and7 for example. An aqueous polymer dispersion is obtained whose dispersedparticles have an average particle diameter of preferably 80 to 200 nm.The average particle diameter of the polymer particles can be determinedby dynamic light scattering on a 0.005% to 0.01% by weight aqueouspolymer dispersion at 23° C. by means of an Autosizer IIC from MalvernInstruments, England. The reported data are all based on the cumulantz-average diameter of the measured autocorrelation function as per ISOstandard 13321.

The mixture of aqueous secondary dispersion and aqueous primarydispersion is used in the present invention for producing paper coatingcompositions. Paper coating compositions, in addition to water,generally comprise pigments, binders and optionally auxiliaries forsetting the requisite rheological properties, for example thickeners.The pigments are typically dispersed in water. The paper coatingcomposition comprises pigments in an amount of preferably at least 80%by weight, for example 80% to 95% by weight or 80% to 90% by weight,based on the total solids content. White pigments are contemplated inparticular. Suitable pigments include, for example, metal salt pigmentssuch as, for example, calcium sulfate, calcium aluminate sulfate, bariumsulfate, magnesium carbonate and calcium carbonate, of which carbonatepigments, more particularly calcium carbonate, are preferred. Thecalcium carbonate may be natural ground calcium carbonate (GCC),precipitated calcium carbonate (PCC), lime or chalk. Suitable calciumcarbonate pigments are available for example as Covercarb® 60,Hydrocarb® 60 or Hydrocarb® 90 ME. Further suitable pigments include,for example, silicas, aluminas, aluminum hydrate, silicates, titaniumdioxide, zinc oxide, kaolin, argillaceous earth, talc or silicondioxide. Suitable further pigments are available for example as Capim®MP 50 (Clay), Hydragloss® 90 (Clay) or Talcum C10.

The paper coating composition comprises as binder the polymers presentin the above-described primary and secondary dispersions. The mostimportant functions of binders in paper coating compositions are to bindthe pigments to the paper and the pigments to each other and to someextent fill voids between pigment particles. For every 100 parts byweight of pigments, the amount of organic binder used (in terms ofbinder solids, i.e. without water or other solvent liquid at 21° C., 1bar) is for example in the range from 1 to 50 parts by weight,preferably in the range from 1 to 25 parts by weight or in the rangefrom 5 to 20 parts by weight.

Optional further binders include natural-based binders, moreparticularly binders based on starch. A binder based on starch is inthis context to be understood as referring to any native, modified ordegraded starch. Native starches can consist of amylose, amylopectin ormixtures thereof. Modified starches may comprise oxidized starch, starchesters or starch ethers. Hydrolysis can be used to reduce the molecularweight of the starch (degraded starch). Possible degradation productsinclude oligosaccharides or dextrins. Preferred starches are cerealstarch, maize starch and potato starch. Particular preference is givento cereal starch and maize starch and very particular preference isgiven to cereal starch.

Paper coating compositions of the present invention may additionallycomprise further addition and auxiliary substances, for example fillers,cobinders and thickeners to further optimize viscosity and waterretention, optical brighteners, dispersants, surfactants, lubricants(e.g., calcium stearate and waxes), neutralizing agents (e.g., NaOH orammonium hydroxide) for pH adjustment, defoamers, deaerators,preservatives (biocides for example), flow control agents, dyes (solubledyes in particular), etc. Useful thickeners in addition to syntheticpolymers (crosslinked polyacrylate for example) include particularlycelluloses, preferably carboxymethylcellulose. Optical brighteners are,for example, fluorescent or phosphorescent dyes, particularly stilbenes.

The paper coating composition of the present invention preferablycomprises an aqueous paper coating composition; water is present thereinparticularly due to the make-up form of the constituents (aqueouspolymer dispersions, aqueous pigment slurries); the desired viscositycan be set by adding further water. Customary solids contents of papercoating compositions range from 30% to 70% by weight. The pH of thepaper coating composition is preferably adjusted to values in the rangefrom 6 to 10, more particularly in the range from 7 to 9.5.

One embodiment of the present invention relates to a paper coatingcomposition wherein the polymers of the emulsion polymer and of thesecondary dispersion are used in an amount of altogether 1 to 50 partsby weight, based on the total amount of pigments, and wherein thepigments are present in an amount of 80 to 95 parts by weight, based ontotal solids content.

The pigments are preferably selected from the group consisting ofcalcium sulfate, calcium aluminate sulfate, barium sulfate, magnesiumcarbonate, calcium carbonate, silicas, aluminas, aluminum hydrate,silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth,talc and silicon dioxide.

The paper coating composition preferably additionally comprises at leastone auxiliary sub-stance selected from the group consisting ofthickeners, further polymeric binders, cobinders, optical brighteners,fillers, flow control agents, dispersants, surfactants, lubricants,neutralizing agents, defoamers, deaerators, preservatives and dyes.

In a preferred paper coating composition the polymeric particles of theemulsion polymer have an average particle size in the range from 80 to200 nm and the polymeric particles of the polymer of the secondarydispersion have an average particle size in the range from 100 to 400nm.

The present invention also provides paper or card coated with a papercoating composition of the present invention and a process for coatingpaper or card, which comprises

-   -   providing a paper coating composition according to the present        invention; and    -   applying the paper coating composition to at least one surface        of paper or card.

The paper coating composition is preferably applied to uncoated basepapers or uncoated card. The amount is generally in the range from 1 to50 g, and preferably in the range from 5 to 30 g (in terms of solids,i.e., without water or other solvent liquid at 21° C., 1 bar) per squaremeter. Coating can be effected by means of customary methods ofapplication, for example via size press, film press, blade coater, airbrush, doctor blade, curtain coating or spray coater. Depending on thepigment system, the paper coating compositions of the present inventioncan be used for the basecoat and/or for the topcoat.

Paper coating compositions according to the present invention have goodperformance characteristics. They have a high binding force and areobtainable in a convenient and inexpensive manner. Papers coated withpaper coating compositions are readily printable in the customaryprinting processes, such as relief printing, gravure, offset, digital,inkjet, flexographic, newsprint, letterpress, sublimation printing,laser printing, electrophotographic printing or a combination thereof.

EXAMPLES

Unless the context suggests otherwise, percentages are always by weight.A reported content is based on the content in aqueous solution ordispersion.

Primary dispersion P1:Styronal® D 809 (50% aqueous polymer dispersion based on carboxylatedstyrene/butadiene copolymer); particle size: 160 nm, glass transitiontemperature 20° C.Primary dispersion P2:Acronal® S 728 (50% aqueous polymer dispersion based on carboxylatedstyrene/butyl acrylate copolymer); particle size: 175 nm, glasstransition temperature 20° C.Secondary dispersion S1:Polypropylene carbonate dispersed in waterMn ca. 80 000 g/molAverage particle size by HDC: 390 nmSecondary dispersion S2Ecoflex® (polyester dispersion in water; aliphatic/aromatic copolyesterbased on terephthalic acid, adipic acid and 1,4-butanediol)Secondary dispersion S3Poligen® WE1 (about 35% strength aqueous polyethylene wax emulsion)

Paper Coating Composition:

The coating slip is prepared in a stirred assembly (Deliteur) into whichthe individual components were fed in succession. The pigments are addedin pre-dispersed form (as a slurry). The other components are addedafter the pigments, the order corresponding to the order in the recitedcoating slip formulation. The final solids content is set by addingwater.Paper coating slip compositions were prepared using mixtures ofdispersion P1 with dispersions S1 to S3 as a binder of the followingcomposition:70 parts by weight of Hydrocarb® 60 slurry (coarse calcium carbonate)30 parts by weight of fine clay (Amazon 88)0.3 part by weight of dispersant (Polysalz S; polyacrylic acid)0.22 part by weight of Sterocoll® FD rheology modifier9 parts by weight of binder (see Table 1)pH set to about 9.0 with aqueous sodium hydroxide solutionBrookfield viscosity about 2000-4000 mPassolids content: 65-66% by weight

TABLE 1 Binder mixing ratios (parts by weight based on solids) ExampleP1 P2 S1 S2 S3 1 2 1 2 1 1 3 1 2 4 2 1 5 1 1 6 1 2 7 1 8 2 1 9 1 1 10 12 11 1 12 1 13 1 14 2 15 1

The coating slip is applied to one side of a paper substrate using apilot-scale coating machine. Coating layer add-on was 10 g/m².

The coated paper was tested for surface resistance using test methodsknown to a person skilled in the art. The following test methods wereused:

IGT dry pick resistanceIGT wet pick resistance

Offset Test

The results are summarized in Table 2.

Measurement of Dry Pick Resistance Using IGT Tester (IGT Dry)

Strips were cut out of the in-test papers and printed using the IGTtester. The printing inks used are specialty test inks from Lorillieux,which transmit different tensile forces. The test strips are fed throughthe press at continuously increasing speed (maximum speed 200 cm/s). Forevaluation, the point at which 10 picks have occurred on the papersurface after the start of printing is determined on the sample printingstrip. The measure reported for dry pick resistance is the speed in cm/spresent at this point during printing and also the test ink used. Thehigher this printing speed at the tenth pick point, the better thequality rating of the paper surface.

Measurement of Wet Pick Resistance Using IGT Tester (IGT Wet)

Strips were cut out of the in-test papers and printed using the IGTtester. The tester was set up such that the test strips are moistenedwith water before printing. The printing inks used are specialty testinks from Lorilleux (No. 3807), which transmit different tensile forces.The print is performed at a constant speed of 0.6 cm/s. Picks from thepaper surface are visible as unprinted areas. To determine wet pickresistance, an ink densitometer is used to determine ink density as a %age of the full hue. The higher the reported ink density, the better thewet pick resistance.

Offset Test:

Samples having a size of 240×46 mm are cut out of the in-test papers inthe longitudinal direction. An appropriate amount of printing ink isapplied to the inking roll and left to run for 1 minute. A printing diskis then inserted and inked for 30 s. The printing speed is 1 m/s. Apaper strip is brought back to the starting position on a printing testsupport with the printed paper strip. After a specified time interval,the printing process is started again without replacing the printingdisk. This operation is repeated more than once. After each printingcycle, the pick on the printed side of the paper strip is assessed byvisual inspection. The table reports the number of cycles before pickingoccurred for the first time. The higher the number of cycles up to theoccurrence of picking, the better the suitability of the papers foroffset printing.

TABLE 2 Measured binding force results Dry pick Wet pick resistanceresistance Offset Example [cm/s] [%] cycles 1 56 31 3.0 2 51 20 3.0 3 459 2.5 4 99 89 3.0 5 87 81 2.5 6 47 21 2.25 7 12 4 1.25 8 56 48 3.0 9 4925 2.75 10 37 14 2.25 11 13 20 1.0 12 40 16 3.25 13 32 11 3.0 14 21 102.0 15 0 9 1.0As the experimental results show, the non-inventive coating slips ofexamples 7, 11 and 15 featuring the straight secondary dispersions asbinders have relatively poor values in respect of the important offsetprinting properties of dry pick resistance. Binder mixtures with primarydispersions display a disproportionately large increase in pigmentbinding capacity starting with a mixing ratio of just 1:2.

1. A process for improving the properties of a paper coating, the process comprising combining a paper composition with a binder comprising a mixture of: (a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of a polyalkylene carbonate, a polyester and a polyethylene; and (b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C., to form an improved paper coating.
 2. The process of claim 1, wherein the emulsion polymer has a glass transition temperature in the range from −10 to +30° C.
 3. The process of claim 1, wherein the at least one polymer of the secondary dispersion has a glass transition temperature in the range from −50 to +50° C.
 4. The process of claim 1, wherein the weight ratio of the emulsion polymer to the polymer of the secondary dispersion is in the range from 1:2 to 2:1.
 5. The process of claim 1, wherein the emulsion polymer is obtained by a free-radically initiated emulsion polymerization from one or more ethylenically unsaturated, free-radically polymerizable monomers selected from the group consisting of a vinylaromatic compound, a conjugated aliphatic diene, an ethylenically unsaturated acid, an ethylenically unsaturated carboxamide, a ethylenically unsaturated carbonitrile, a vinyl ester of an saturated C₁ to C₂₀ carboxylic acid, an ester of acrylic acid or methacrylic acid with a monohydric C₁ to C₂₀ alcohol, an allyl ester of a saturated carboxylic acid, a vinyl ether, a vinyl ketone, a dialkyl ester of an ethylenically unsaturated dicarboxylic acid, N-vinylpyrrolidone, N-vinylpyrrolidine, N-vinylformamide, an N,N-dialkylaminoalkylacrylamide, an N,N-dialkylaminoalkyl acrylate, an N,N-dialkylaminoalkyl methacrylate, a vinyl halide, an aliphatic hydrocarbon having 2 to 8 carbon atoms and one or two double bonds, and a mixture thereof.
 6. The process of claim 1, wherein the emulsion polymer is selected from the group consisting of (i) a copolymer from 19.8 to 80 parts by weight of at least one vinylaromatic compound, 19.8 to 80 parts by weight of at least one conjugated aliphatic diene, 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomer sum to 100; (ii) a copolymer from 19.8 to 80 parts by weight of at least one vinylaromatic compound, 19.8 to 80 parts by weight of at least one acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, or their mixture, 0.1 to 10 parts by weight of at least one ethylenically unsaturated acid, and 0 to 20 parts by weight of at least one other monoethylenically unsaturated monomer, wherein the parts by weight of monomer sum to 100; (iii) a copolymer from vinyl acetate and at least one (meth)acrylate monomer selected from C1 to C10 alkyl acrylates and C1 to C10 alkyl methacrylates, and (iv) an ethylene-vinyl acetate copolymer.
 7. The process of claim 1, wherein the emulsion polymer is constructed from butadiene or mixtures of butadiene and styrene to an extent of at least 60% by weight or from C1 to C20 alkyl (meth)acrylates or mixtures of C1 to C20 alkyl (meth)acrylates and styrene to an extent of at least 60% by weight.
 8. The process of claim 1, wherein the emulsion polymer is prepared from (A1) 19.8 to 80 parts by weight of at least one vinylaromatic compound, (B1) 19.8 to 80 parts by weight of at least one conjugated aliphatic diene, (C1) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and (D1) 0 to 20 parts by weight of at least one further monoethylenically unsaturated monomer other than said monomers (A1) to (C1); or (A2) 19.8 to 80 parts by weight of at least one vinylaromatic compound, (B2) 19.8 to 80 parts by weight of at least one monomer selected from C1 to C18 alkyl esters of acrylic acid and C1 to C18 alkyl esters of methacrylic acid, (C2) 0.1 to 15 parts by weight of at least one ethylenically unsaturated acid, and (D2) 0 to 20 parts by weight of at least one further monoethylenically unsaturated monomer other than said monomers (A2) to (C2), wherein the parts by weight of monomers (A1) to (D1) or (A2) to (D2) sum to 100 in either case.
 9. The process of claim 8, wherein: said monomers (D1) and (D2) are included in amounts of 0.1-15 parts by weight; the vinylaromatic compound is selected from the group consisting of styrene, methylstyrene and their mixture; the conjugated aliphatic diene is selected from the group consisting of 1,3-butadiene, isoprene and their mixture; and the ethylenically unsaturated acid is selected from one or more compounds of the group consisting of acrylic acid, methacrylic acid, itaconic acid, maleic acid, fumaric acid, crotonic acid, vinylacetic acid, vinyllactic acid, vinylsulfonic acid, styrenesulfonic acid, acrylamidomethylpropanesulfonic acid, sulfopropyl acrylate, sulfopropyl methacrylate, vinylphosphonic acid and salts thereof.
 10. A paper coating composition, comprising (a) an aqueous secondary dispersion of at least one polymer selected from the group consisting of a polyalkylene carbonate, a polyester, and a polyethylene, (b) an aqueous primary dispersion of at least one emulsion polymer having a glass transition temperature of below 50° C., and preferably in the range from −10 to +30° C., and (c) inorganic pigments.
 11. The paper coating composition of claim 10, wherein the polymeric particles of the emulsion polymer have an average particle size in the range from 80 to 200 nm and the polymeric particles of the polymer of the secondary dispersion have an average particle size in the range from 100 to 400 nm.
 12. The paper coating composition of claim 10, wherein the pigments are selected from the group consisting of calcium sulfate, calcium aluminate sulfate, barium sulfate, magnesium carbonate, calcium carbonate, silicas, aluminas, aluminum hydrate, silicates, titanium dioxide, zinc oxide, kaolin, argillaceous earth, talc and silicon dioxide, and wherein the paper coating slip optionally further comprises at least one additive selected from the group consisting of thickeners, further polymeric binders, co-binders, optical brighteners, fillers, flow control agents, dispersants, surfactants, lubricants, neutralizing agents, defoamers, deaerators, preservatives and dyes.
 13. The paper coating composition according to claim 10, wherein: the polymers of the emulsion polymer and of the secondary dispersion are present used in total in an amount of 1 to 50 parts by weight, based on the total amount of pigments, and the pigments are present in an amount of 80 to 95 parts by weight, based on total solids content.
 14. A paper or card coated with a paper coating composition of claim
 10. 